Asphalt is a common material utilized for the preparation of paving materials. While the material is suitable in many respects, it inherently is deficient in some physical properties which it would be highly desirable to improve. Efforts have been made in this direction by addition of certain conjugated diene rubbers, ethylene containing plastics like EVA and polyethylene, neoprene, resins, fillers and other materials for the modification of one or more of the physical properties of the asphalt binder. Each of these added materials modifies the asphalt in one respect or another but certain deficiencies can be noted in all modifiers proposed. For example, some of them improve only the high temperature performance of asphalt, some only improve the low temperature performance of asphalt, while some lack thermal stability or mixing stability with asphalt.
Since the late 1960s, styrene-butadiene rubber and styrene-rubber block copolymers such as styrene-butadiene-styrene and styrene-isoprenestyrene block copolymers have been used to dramatically improve the thermal and mechanical properties of asphalts. Practical application of the rubber addition approach requires that the blended product retain improved properties and homogeneity during transportation, storage and processing. Long term performance of elastomer-modified asphalts also depends on the ability of the blend to maintain thermal and chemical stability.
To be suitable for paving materials, the asphalt-block copolymer mixtures should meet the following requirements:
(a) The block copolymer must be mixable in asphalt and stay mixed during subsequent processing.
(b) The mixture must have the right rheological (flow) properties to prevent rutting which is the permanent deformation of a road caused by repetitive traffic loads. Viscosity is important but elasticity is the most important aspect since the material must be able to recover rather than just resist deformation. This characteristic is most important in warm climates.
(c) The mixture must have good low temperature properties, i.e. resistance to cracking. As a road cools, stresses develop because it cannot shrink uniformly and eventually this will cause cracking. Traffic-caused stressed also contribute. The block copolymer will lower the temperature at which cracking will occur. This characteristic is more important in cold climates.
These block copolymers are necessary to obtain improved bituminous compositions, specifically for paving, and specifically for hot mix asphalt concrete (HMAC). The polymer is added to the bitumens for hot mix asphalt concrete to improve its resistance to deformation caused by traffic loading especially at warm times of the year (rutting resistance), as well as improved resistance to cracking caused by both traffic and thermally induced loads, especially at cold times of the year. It is also necessary that the polymer by easy to mix with the bitumen and that the two do not demix during storage or processing. The polymers must be oxidatively stable during processing and during long term aging on the road.
Temperature susceptibility of a polymer modified asphalt is a major consideration. Ideally, one would want a binder (asphalt and polymer) which would be "hard" and elastic at elevated temperatures to resist permanent deformation and "soft" and strong at low temperatures to resist cracking. In other words, the ideal system should not only exhibit enhanced resistance to deformation but also should resist cracking.
At the present time, unhydrogenated block copolymers are being used in paving applications. Unhydrogenated block copolymers have certain disadvantages which can cause problems when used in applications such as these. Such disadvantages include poor stability of the block copolymer during blending and storage of the bituminous composition and poor long term stability when the bituminous composition is exposed to the elements (by stability we mean resistance to degradation).